Vertical-cavity surface-emitting lasers (VCSELs) are used as light sources in a variety of electronic applications including fiber optic communications, laser printing, and optical data storage. A VCSEL is an injection diode laser where the laser oscillation and output occur normal to a semiconductor pn junction plane. In edge-emitting laser diodes, the laser oscillation and output occur in the semiconductor pn junction plane. VCSELs have many advantages compared with edge-emitting laser diodes. These advantages include a low divergence circular output, single longitudinal mode operation, and high two-dimensional packing density.
All lasers use the principle of amplification of electromagnetic waves by stimulated emission of radiation. The term laser is an acronym for light amplification by stimulated emission of radiation.
The process of stimulated emission can be described as follows. When atoms, ions, or molecules absorb energy, they can emit light spontaneously (as in an incandescent lamp) or they can be stimulated to emit by a light wave. If a collection of atoms is pumped so that more are initially excited than unexcited, then an incident light wave will stimulate more emission than absorption, and there is net amplification of the incident light beam. This is the way a laser amplifier works.
A laser amplifier can be made into a laser oscillator by arranging suitable mirrors on either end of the amplifier to form a resonator. Thus, the essential parts of a laser oscillator are an amplifying medium, a source of pump power, and a resonator. Radiation that is directed straight along the axis bounces back and forth between the mirrors and can remain in the resonator long enough to build up a strong oscillation. Radiation may be coupled by making one mirror partially transparent so that part of the amplified light can emerge through it.
The fundamental light-producing mechanism in an injection diode laser, such as a VCSEL, is the recombination of excess conduction-band electrons and valence-band holes. This recombination operation takes place by pumping excess carriers across a junction. That is, excess electrons are injected from a semiconductor n-layer and excess holes from a semiconductor p-layer into an active waveguide region, where they recombine, via stimulated emission, producing the desired gain. The lasing threshold is reached when optical loss is balanced by optical gain.
Many applications of semiconductor lasers require dynamic stabilization of the light output against variations in the external environment. It is also necessary to monitor long-term drift in the laser and its drive circuitry. In most commercially available diode lasers, this is done by using a discrete external monitoring photodiode chip. The photocurrent generated by the external detector is used in a feedback circuit to adjust the laser injection current.
Recently, it has been proposed to substitute discrete external monitoring photodiode chips with monolithic photodiodes for the use with VCSELs. In these schemes, photodiodes are placed either on the top, on the bottom, on the side, or along the perimeter of the VCSEL. In this location, the photodiode directly intercepts and samples the laser emission as it exits the device.
While the monolithic approach avoids many of the problems associated with the use of a separate monitoring photodiode chip, its performance is far from ideal. The primary problem with these devices is that they detect too much spontaneous emission. In addition, they are susceptible to ambient light. Consequently, the performance of the proposed monolithic photodiodes is inferior to that of a separate monitoring photodiode chip. Thus, it would be highly desirable to develop an improved monolithic photodetector for use in VCSELs.
In addition to the need for improved monolithic photodetectors for use in VCSELs, there is a need for improved modulation of VCSELs at high frequencies. Primary characteristics of interest in VCSEL modulation include high speed, high modulation depth, and low frequency chirp. Depending on the modulation technique and the application involved, any combination of these features may be manifested. For example, an external modulator integrated with a VCSEL eliminates frequency chirping since the modulating element is outside the lasing cavity. However, if the absorption layer is thin (i.e., a single quantum well), then the speed is high but the small single-pass absorption leads to low modulation depth. If the absorption layer is thick, the thickness required for a bulk layer would be very large, requiring large voltages to achieve the same electric field as that across a single quantum well since the interaction length in a vertical cavity is much shorter than that in an in-plane laser. Multiple quantum wells would number in the tens or hundreds--an impractical growth.
Direct current modulation has been demonstrated for high speeds and high modulation depths. However, the changes in carrier density introduce frequency chirp, which may not be suitable for applications where frequency stability is required.
One technique to obtain modulated light output from a VCSEL is to generate self-pulsation in the device. A self-pulsating VCSEL has an AC output derived from a DC input. Known self-pulsating lasers are edge-emitters. It would be highly desirable to develop a self-pulsating VCSEL because the surface-normal geometry would facilitate two-dimensional array configurations and wafer-scale fabrication. In addition, the circular beam output of the VCSEL could be exploited.
Although self-pulsation in VCSELs has been previously analyzed and observed with repetition rates up to the MHz range, no VCSEL has yet been observed to self-pulsate at high frequency or with a controllable saturable absorber. In the prior art, the self-sustained oscillations have been observed by introducing, intentionally or inadvertently, saturable absorbing centers surrounding the device cavity. In view of the foregoing, it would be highly desirable to provide a VCSEL with improved self-pulsation through a wavelength dependent intracavity quantum-well absorber.
Lasers are used in the prior art for optical pick-up detection. However, current optical disk readout schemes use an edge-emitting laser as the optical source and a separate external photodetector. It would be highly desirable to integrate the optical source and detector into a single device. Such an approach would eliminate noise due to reflections from unnecessary bulk optics and also minimize the physical size of the device. It would also be desirable to use an end-emitting laser, the circular beam output of which would allow the the output beam to be focussed to a spot size that is smaller than that of an edge-emitting device. Focusing the beam to a smaller spot size would increase the allowable information density on the optical disk.
In sum, it would be highly desirable to develop an improved VCSEL to overcome the foregoing limitations associated with prior art VCSEL technology.